Which horizon is known as the humus layer or topsoil?

The correct answer identifies Horizon A as the humus layer or topsoil. This horizon is rich in organic matter, including decomposed plant and animal materials, which contribute to its nutrient-rich characteristics. This layer is crucial for supporting plant life because it holds water and provides essential nutrients. The presence of organisms like earthworms and various microorganisms in this layer further enhances its fertility and structure, promoting a healthy soil ecosystem.

In contrast, other horizons do not have the same designation: Horizon O, for example, typically consists of organic matter yet is more related to the accumulation of surface litter, while Horizon C represents partially weathered rock and parent material, lacking the organic richness found in Horizon A. Horizon B is often referred to as subsoil and has less organic content compared to the topsoil. Understanding these distinctions is vital when studying soil profiles and their agricultural implications.